River and sea lampreys, which lived 200 million years before the dinosaurs, are the world's oldest living vertebrates, the largest of which can grow up to 1 metre in length. These "living fossils" are particularly distinctive due to their unnerving appearance: rather than jaws, lampreys have a ferocious-looking disc of razor teeth.

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Once common in British waters, lampreys are now an endangered species across Europe, after pollution caused by industrialisation diminished their numbers. However, it now seems lampreys are reappearing in their old habitats, as rivers see their lowest pollution levels for more than 100 years.

The removal of some river structures, such as mill weirs, has also helped to boost lamprey migration. The Environment Agency has also been trialling new techniques to help the species thrive once again. Lamprey "tiles" -- low-cost cones that enable the fish to swim upstream using their sucker-like mouths as "anchors" -- have helped them return to their original spawning grounds, and have seen a surge in the lamprey population on the Derwent in Yorkshire.

Simon Toms, Environment Agency fisheries expert, explained: "For the last 200 hundred years, some rivers have not been capable of supporting lamprey species as a result of water quality, poor habitat and manmade barriers. "Now that water quality has improved and some of these barriers have been removed we are seeing lampreys return to the upper reaches of rivers such as the Ouse, Trent, and Derwent, where they were absent as recently as 30 years ago."

Lampreys are important for processing vital river nutrients and providing a food source for other fish. They were also considered a gourmet delicacy throughout history -- it is even said that King Henry I died after over-indulging in them. More recently, our very own Queen Elizabeth II was sent a lamprey pie for her Diamond Jubilee in 2012, though the fish were sourced from North America.